Suppression of echoes and singing in four-wire circuits



Dec. 24, 1929. I H. NYQUIST 1,740,621

SUPPRES'SION OF mamas AND swarm m FOUR-WIRE'CIRCUITS Original Filed Oct. 22, 192'? Repeater r IN VEN TOR.

ATTORNEY Patented Dec. 24, 1929 PATENT OFFICE nanny rtvours'r, or MILLBURN, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YoRK SUPPRESSION OF ECHOES AND SINGING IN IEOUR-WIRECIRCUITS Application filed October 22, 1927, Serial No. 228,061. Renewed September 15, 1928.

This invention relates to two-way communication systems, and more particularly to two-way telephone systems which include, between two-w1re lines, a four-wire circuit in one for transmission in the opposite direction. While the invention is applicable to any four-wire circuit lying between two two-wire The term four-wlre circuit is ordinarily used, in connection with systems including a radio link,'to denote the part of the system which connects the two distant two-wire lines and includes the radio link; this term' fourwire circuit will be used hereinafter, however, with a view to a clear description'of the invention, to denote that four-wire section of ahalf ofthe complete-system which lies between one two-wire line and one end of the radio link. It will readily be understood that if the system to which the invention is to be applie is an all-wire system, the radio link, as referred to herein, is replacedby a length of the fourwire circuit extendmg over the whole-distance between the two-wire invention designed to overcome certa1n unlines.

In a telephone system of this class, in which the two paths of the four-wire circuit are adapted for transmission in opposite directions, it is, under certain conditions, a requirement, if the best possible results are to be secured, that one path of the four-wirecircuit be normally blocked and consequently normally disabled. This arrangement serves to prevent singing in the circuit in its normal condition. In addition, in association with each transmission path of the four-wire circuit there must'be provided means responsive to waves in this path for blocking the transmission over theother path, in order to secure the suppression of echoes returning from the two-wire line over that other path.

The arrangement of the four-wire circuit which is ordinarily preferable is that in which the outgoing paththat is, the one adapted for transmission to the radio link-is normally blocked,-while the path adapted for having one path adapted for transmission direction and another path adapted course, necessary to provide means responsive to voice waves in the outgoing path for clearing this transmission path.

In the arrangement of four-wire circuits which hasbeen found comparatively successful in all-wire systems, an amplifier-detector device is associated with each path of the .circuit. Voice waves in the transmitting (or outgoing) path enter the associated amplitier-detector, and the output causes the operation of a relay or'r-elays to place a short circuit on, or otherwise block, the receiving path, and also to clear the transmitting path. Voice waves in the receiving path, provided this path is not blocked at the time, enter theassociated amplifierr'detector, and the'output causes-the operation of a relay or relays to block thetransmitting path.

An improvement inthe suppression of echoes and singing in four-wirecircuits residing chiefly in simplification of the voice operated apparatus, is disclosed in the United States Patent No. 1,653,73 8 to H. G. Silent dated December 27, 1927. As will appear more-clearly hereinafter, the applicants invention is an improvement over the Silent satisfactory features of the operation of that arrangement. In the Silent arrangement, a relaywhich controls the clearing and blocking of the paths of the four-wire circuit is connected through suitable apparatus to both transmission paths, and the eifect of echo currents in the outgoing path on this relay is oii set by the effect of current taken directly from the incoming path.

The principal object of the applicants invention is to improve the balancing of these two effects, the first caused by the echoes in one path and the second caused by the incoming currents in the other path.

Another object of this invention is to pro vide, in combination with the simplified form of voice operated apparatus, the lock-out feature which will enable the distant speaker to hold his control of the circuit while he'. is

talking.

The invention will be. more clearly understood when the following description is read with reference to the'accompanying drawing, whichshows diagrammatically the essential transmission features of the applicants improved arrangement.

The four-wire circuit is connected at its left end to a two-wire line through the usual hybrid coil H, the two-wire line being balanced by the usual network. The lower path of the four-wire circuit is adapted for transmission over a radio link (at the right) andv "and the detector D The amplifier A and he detector D are associated with the re:

ceiving path of the circuit.

A polar relay S has one winding in the plate circuit of the detector D and a second winding in-the plate circuit of the detector D These two windings are differentially connected, so that a current flow in the lower v y the transmitting winding tends to operate the relay and-current in the upper winding opposes this opera- I tion Thebattery B is connected as shown. The operation of the relay S causes the operation'of the relays S and S The transmitting path of the four-wire circuit is normally blocked by a short circuit which 'can be removed by the operation of relay S The reception path is normally clear but will be blocked by a short circuit. upon the operation of relay I When the four-wire circuit is quiet and voice waves pass over the two-wire line to path of the four-wire circuit, the waves enter the amplifier-detector A -D This amplifier-detector is operated, and the output current causes,the operation of relays S S and S there being no current flow in the upper winding of relay S The operation of therelays blocks the receiving .path and clears the transmitting path.

When, however, noise or voice waves enter the receiving path of the four-wire link and are transmitted on to the two-wire line, they act upon the amplifier-detector A,-D and cause a current flow in the upper winding of relay S which opposes the operation of the relay. If these waves are reflected in the twowire-line back over the transmitting path of the four-wire ciri. uit, the echoes act upon the amplifier-detector A -D setting up a cur-,

rent in the plate circuit of the detector which tends to operate the relay S The magnitude of these echoes depends upon the closeness with which the two-wire line is balanced by its-network. The twoamplifier-detectors similar, a resistance are given the proper sensitivities, with due ation of the transmitting relays'by noise in the receiving side of the tour-wire circuit, and, consequently, there is at least a reduced interference with the passage of voice waves from the two-Wire line over the transmission .path of the four-Wire circuit.

' Small delays in the; echoes are taken care of by the insertion of. a resistance R and a condenser C in the circuit of the amplifierdetector ri -D This arrangement has the effect of delaying the establishment and the removal of the current in the upper winding of the relay S In order that the characteristics of thetwo amgiifier-detectors may be and a condenserjC are introduced into the plate circuit of the detector 1),. This arrangement does not produce a hang-over effect in the circuit.

The circuits as described thus far are substantially those of the Silent disclosure identified above.

It is to be expected,- however, in the case of Silent, the applicant associatesa potenti- I ometer P, or any other suitable device for adjusting the neutralizing current, with the input to the amplifier A This device P may be operated by a technical operator to adjust the current acting-on the detector D and the upper winding of relay S so that it will exactly offset in that relay the effect produced in the lower winding-by the attenuated echo currentsin the outgoing transmission Furthermore, the applicant associateswith the path adapted for transmission from the radio link to the two-wire-line anadditional amplifier-detector as shown in'the drawing. This amplifier-detector controls a relay S which, upon operation, short-circuits the out-.

going transmission path, as specifically disclosed in the drawing, for instance; It is to be understood that the essential function of the additional amplifier-detector and the relay S controlled thereby is to prevent the operation of the amplifier A by voice waves traveling from the two-wire line over the transmitting path to the radio transmitter; this amplifier-detector operation may be pre vented by a short-circuit effected at any point in the path over which currents in the transmitting channel must travel in order to reach and aifect the amplifier A With this adpath is short circuited by the operation of the relay S and the voice from the two-wire line neither passes on to the radio link nor in any way affects the amplifier-detectorapparatu's controlling the relay S and the relays S and S While a specific circuit arrangement has been described for the purpose of clear illustration, it is to be understood that the scope of the applicants invention is not limited thereby but is defined by the appended claims.

What is claimed is:

1. h a two-way telephone circuit having a first path adapted for transmission in one direction and a second path adapted tortransmission in the opposite direction, the first path being normally blocked at a point rela-- tively near its outgoing end and the second path being normally cleared, an amplifier-detector associated with the first path, an amplifier-detector associated with the second path, a relay having two windings diilerentially connected and included one in the output circuit of each of saidamplifier-detectors, means associated with the input circuit of the amplifier-detector of the secondpath for adj usting the current therein,'means responsive to the operation of said relay for clearing the first path and simultaneously blocking the second path, and means responsive to voice currents in the second path for blocking the first path at a point relatively distant from, its outgoing end.

2. In a two-way telephone system includ- 7 ing a four wire circuit having a two-wire line connected to one end thereof, the path of the four-wire circuit which is adapted for transmission from the two-wire line being normal- 1y blocked at a point relatively near its outgo-' ing end and the path of said circuit which is adapted for transmission to the two-wire line being normally cleared, an amplifier-dd tector associated with each path, a relay having two windings difierentially connected and included one in the output circuit of each amplifier-detector, means associated with the input circuit of the amplifier-detector of the normally clearedpath for adjusting the current therein, means responsive to the operation of said relay for clearing the normally blocked path and simultaneously blocking the normally cleared path, and means responsive to voice currents in the normally cleared path for blocking the normally blocked path at a point relatively distant from its outgoing end.

'3v In a two-way telephone circuit having afirst path adapted for "transmission in one direction and a second path adapted for transmission in the opposite direction, the first path being normally blocked and the sec ond path being normally cleared, an amplitier-detector associated with the first path, an

amplifier-detector associated with the second path, a relay having two windings differentia-lly connected and included one in the output circuit of each of said amplifier-detectors, means associated with the input circuit of the amplifier-detector of the second path for adjusting the current therein, means responsive to the operation of said relay for clearing the first path and simultaneously blocking the second path, and means responsive to voice currents in the second path for preventing currents in the first path from op- I crating the amplifier-detector associated with the first path,

4. In a two-way telephonesystem including a tour-wire circuit having a two-wire line connected to one end thereof, the pathof the fourwire circuit which is adapted for transmission from the two-wire line being normally blocked and the path of said circuit which is adapted for transmission to the two-wire line being normally cleared, and an amplifier-detector associated with each path, a relay having two windings diiierentially connected and included one in the output circuit of each amplifier-detector, means associated with the input circuit of the amplifier-detector of the normally cleared path for adjusting the current therein, means responsive to the operation of said relay for clearing the normally blocked path and simultaneously blocking the normally cleared path, and means responsive to voice currents in the normally cleared path for preventing currents in the normally blocked path from operating the amplifierdetector associated with the normally bloc'ked path.

In testimony whereof, I have signed my name to this specification this 20th day of October 1927.

- HARRY NYQUIST. 

